Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger comprises an exhaust gas driven turbine wheel, mounted on a rotatable shaft, within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel, mounted on the other end of the shaft, within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housing.
The turbine stage of a conventional turbocharger comprises: a turbine housing defining a turbine chamber within which the turbine wheel is mounted; an annular throat defined in the housing between facing radially extending walls and arranged around the turbine chamber to form an inlet passage; and an outlet passage extending from the turbine chamber. These components communicate such that pressurised exhaust gas admitted to the housing flows through the throat to the outlet passage via the turbine chamber and rotates the turbine wheel. It is known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the throat so as to deflect gas flowing through the throat towards the direction of rotation of the turbine wheel.
The turbines of known turbochargers may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the throat can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied in line with varying engine demands.
In one known type of variable geometry turbine, an axially moveable wall member defines one wall of the throat. The position of the movable wall member relative to a fixed facing wall of the throat is adjustable to control the axial width of the throat. Thus, for example, as exhaust gas flow through the turbine decreases, the throat width may be decreased to maintain the gas velocity and optimise turbine output. The axially movable wall member may be a “nozzle ring” that is provided with vanes that extend into the throat and through orifices provided in a “shroud plate” that defines the fixed facing wall of the throat, the orifices being designed to accommodate movement of the nozzle ring relative to the shroud. Typically the nozzle ring may comprise a radially extending wall (defining one wall of the throat) and radially inner and outer axially extending walls or flanges that extend into an annular cavity behind the radial face of the nozzle ring. The cavity is formed in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing) and accommodates axial movement of the nozzle ring. The flanges may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. In one common arrangement the nozzle ring is supported on rods extending parallel to the axis of rotation of the turbine wheel and is moved by an actuator, which axially displaces the rods. In an alternative type of variable geometry turbocharger, the nozzle ring is fixed and has vanes that extend from a fixed wall through orifices provided in a moving shroud plate.
In another type of variable geometry turbine known as a swing vane turbocharger, the inlet size (or flow size) of the turbine is controlled by an array of movable vanes positioned in the turbine inlet. Each vane can pivot about an axis extending across the inlet parallel to the turbocharger shaft and aligned with a point approximately half way along the length of the vane. A vane actuating mechanism is linked to each of the vanes and is displaceable in a manner which causes each of the vanes to move in unison, such a movement enabling the cross sectional area available for the incoming gas and the angle of approach of the gas to the turbine wheel to be controlled.
It is known to provide a turbocharger turbine with a valve-controlled bypass port referred to as a wastegate, to enable control of the turbocharger boost pressure and/or shaft speed. A wastegate valve (typically a poppet type valve) is controlled to open the wastegate port (bypass port) when the boost pressure of the fluid in the compressor outlet reaches a pre-determined upper limit, thus allowing at least some of the exhaust gas to bypass the turbine wheel. Typically the wastegate port opens into a wastegate passage which diverts the bypass gas flow to the turbine outlet or vents it to the atmosphere. The wastegate valve may be actuated by a variety of means, including electric actuators, but is more typically actuated by a pneumatic actuator operated by boost pressure delivered by the compressor wheel.
Some known internal combustion engines include Exhaust Gas Recirculation (EGR). EGR is used to reduce nitrogen oxide (NOx) emissions of an internal combustion engine. EGR works by recirculating a portion of an exhaust gas produced by the internal combustion engine back to the engine cylinders, usually via the engine intake manifold. Recirculating a portion of the exhaust gas results in a reduction in temperature of the combustion which occurs in the engine cylinders. Because NOx production requires a mixture of nitrogen and oxygen (as found in the air) to be exposed to high temperatures, the lower combustion temperatures resulting from EGR reduces the amount of NOx generated by the combustion. In some known internal combustion engines a variable geometry turbine assembly (which forms part of a turbocharger) is used to increase the pressure (also known as back pressure) of the exhaust gas by partially closing the throat. This creates a pressure differential between the exhaust gas and the engine intake such that the exhaust gas will flow via an exhaust gas recirculation channel to the engine intake. However, the creation of back pressure by the variable geometry turbine can impair the operating performance of the internal combustion engine.
Exhaust gas is generally admitted to the throat of a turbocharger turbine through an inlet volute provided within the turbine housing. The inlet volute has a volute passage which spirals radially inwards from a first end to a second end and terminates at the throat. Exhaust gas from the exhaust manifold of an engine enters the volute passage at the first end, and emerges at the throat at significant angular velocity. The volute passage generally decreases in cross section along its length, so as to increase the velocity of the exhaust gas flow therethrough (and thereby increase the amount of energy which can be extracted by the turbine wheel) and/or to increase the pressure in the volute passage so that exhaust gas is urged out of the passage and into the throat.
Though some turbines utilize a single inlet volute, known turbines such as double flow turbines and twin flow turbines utilize two inlet volutes each including a separate volute passage. The two volute passages are separated by a dividing wall and each has a separate throat. The two passages' throats meet at an inlet passage radially adjacent to the turbine, with different portions of the inlet passage being supplied by the different volute passages. In the case of a twin flow turbine each volute passage supplies a different axial portion of the inlet passage, and in the case of a double flow turbine each volute passage supplies a different circumferential portion of the inlet passage. In other words, in a double flow turbine the two volute passages meet the inlet passage in the same plane, whereas in a twin flow turbine the two volute passages meet the inlet passage in axially adjacent planes.
One advantage of twin flow and double flow turbines is that they allow the segregation of the exhaust gas flows from the engine cylinders which flows would otherwise interfere with each other. Where exhaust from all cylinders feeds a single volute passage, all engine cylinders are connected together by the exhaust manifold. An exhaust gas flow pulse from a first cylinder at the end of its firing stroke and the start of its exhaust stroke can therefore increase the local pressure in the exhaust manifold near a second cylinder which is at the end of its exhaust stroke and the start of its intake stroke (i.e. during its overlap period, in which the intake and exhaust valves of that cylinder are both partially open so that exhaust scavenging can occur), preventing full expulsion of exhaust gas therefrom. In twin flow or double flow turbines, however, this first cylinder can be connected to one volute passage and the second cylinder can be connected to the other. The exhaust flow from these two cylinders is therefore partitioned (by the dividing wall between the volute passages) until it enters the turbine inlet passage. This reduces or eliminates interference with exhaust scavenging processes. This more efficient use of scavenging decreases exhaust gas temperatures (and therefore NOx production), and improves turbine efficiency (thereby reducing turbo lag and increasing boost pressures).
Double flow and twin flow turbines may also provide benefits in relation to EGR. By increasing the number of engine cylinders connected to one of the volute passages, and/or by reducing the cross sectional area of the passage, the exhaust pressure in that volute passage can be increased. This allows the local pressure in one volute passage to be increased to the point where (by connecting the exhaust recirculation channel to this passage) the recirculated exhaust can be supplied at sufficient pressure with a smaller rise in the overall exhaust pressure (and thus a smaller negative effect on engine performance).
In conventional turbochargers, the or each inlet volute has a tongue which projects along a longitudinal axis running substantially within a plane that is normal to the turbine axis. The tongue projects between, and acts to partition, the second end of the volute passage from a part of the passage immediately radially adjacent thereto. While many volute passages do not rotate far beyond 360° around the turbine axis, in figurative terms the tongue can be considered to separate at least the end of the radially innermost ‘coil’ of the passage (i.e. at least the second end of the passage) from the penultimate coil. The tongue terminates in a longitudinally distal tip, which is conventionally positioned radially adjacent to the turbine wheel to provide minimal clearance therewith, and acts to direct working fluid in the second end of the passage into the turbine wheel. In conventional turbines the exhaust gas runs into the turbine wheel radially (i.e. with no axial velocity component), so conventional turbine wheels are designed to be most efficient when their inflow has no axial component. Conventionally, therefore, the lateral centerline of the tongue tip is positioned transverse to this direction (i.e. parallel to the turbine axis) so that the radially inner surface of the tongue tip urges fluid into the turbine wheel accordingly (rather than imparting an axial velocity component, as it would if aligned at an angle to the turbine axis). Further, the casting processes by which the turbine housing is manufactured may be more simple if the lateral centerline of the tongue tip is parallel to the turbine axis.
A key parameter in turbine design is the swirl angle (also known as whirl angle), which is the angle between the radial direction and the direction in which fluid enters the turbine wheel. For instance, if fluid enters a turbine radially then it has a swirl angle of zero, and if it enters a turbine wheel tangentially it has a swirl angle of 90°. The swirl angle in the turbine of a turbocharger is typically between around 20° and around 40°. The swirl angle at a particular angular position about the turbine axis can be defined as:
  α  =      tan    ⁡          (                                    A            wheel                    ⁢                      /                    ⁢                      r            wheel                                                A            passage                    ⁢                      /                    ⁢                      r            passage                              )      Where α is swirl angle, Awheel is the area of the wheel (in circumferential cross section), rwheel is the radial distance to the centroid of Awheel, Apassage is the area of the volute passage at the angular position in question and rpassage is the radial distance to the centroid of that area.
In some applications, Apassage/rpassage (hereafter referred to merely as ‘A/r’), preferably decreases linearly around the turbine axis. This can be useful in controlling the swirl angle so as to optimize the mass flow of working fluid into the turbine. For most angular positions about the circumference of the turbine wheel this can be achieved by adjusting the size and shape of the volute passage. However, problems can arise in the region of the tongue tip. As working fluid running along the volute passage passes the tip of the tongue there is a sudden increase in the area of the passage and the shape of that area (since the tongue no longer occupies any space in the passage). This can lead to a sudden change in the A/r and therefore in the swirl angle. This localized change in swirl angle can create a localized area of high/low force on the turbine wheel around its circumference. This, in turn, can reduce the efficiency of the turbine (for instance by inducing vibration of the turbine wheel) and/or lead to premature failure (for instance from fatigue due to a point on the turbine undergoing increased cyclic loading as it continually travels around the turbine axis and through the localized region). For the reasons discussed below, this problem can be particularly acute in turbochargers where the volute is axially asymmetric (such as twin flow turbines and some double flow turbines), which can lead to such turbines being rejected for use in applications to which they would otherwise be well suited.